EP-A-0361927 describes such a pump but gives no hint as to how to solve the difficulty which comes when attempting to use the principle behind operating a fluid pump for operating a motor, namely how to open the valves against a positive pressure. To use poppet valves in a motor one must either apply a large force to them or supply carefully controlled timing pulses so that each valve only moves when the pressures on opposite sides of it are substantially equal. This specification is concerned with the latter approach in the case of the application of the invention to a motor.
The operational cycle of an hydraulic motor consists of two phases; the first is the power stroke, where the working chamber is exposed to the high-pressure manifold until it is forced to its maximum volume position. The second phase is the exhaust stroke where the valving is rearranged to connect the chamber to the low-pressure manifold and the volume is driven back to the minimum volume position. Normally, commutating ports are used to make the appropriate manifold/chamber connections. These are acceptable unless the standard of filtration is low since the clearance of the interfaces at the commutating ports must be kept small for leakage reasons. The commutating port design also requires a rather delicate design trade-off between leakage and shear losses which are completely eliminated with poppet valves.